1. Field of the Invention
The present invention relates to the field of plant biotechnology. In particular, the invention relates to methods for producing transgenic plants and plant cells by using non-Agrobacterium bacterial species.
2. Description of Related Art
Agrobacterium spp., members of the Rhizobiales, are common soil bacteria, along with Rhizobium spp., Mesorhizobium spp., Sinorhizobium spp., and related species and genera. A number of wild-type and disarmed (non-pathogenic) strains of Agrobacterium tumefaciens and Agrobacterium rhizogenes harboring Ti or Ri plasmids can be used for gene transfer into plants. Phytohormone synthesis genes located in the T-DNA of wild type Agrobacteria harboring a Ti or Ri plasmid are expressed in plant cells following transformation, and cause tumor formation or a hairy root phenotype depending on the Agrobacterium strain or species. Importantly, T-DNA of Agrobacteria can be engineered to replace many of its virulence and pathogenicity determinants with “genes of interest” while retaining the ability to be transferred into a plant cell and integrated into a plant genome. Strains containing such “disarmed” Ti plasmids are widely used for plant transformation.
The mechanism of T-DNA transfer to plant cells by Agrobacterium has been well documented. Briefly, the T-DNA is delimited by two border regions, referred to as right border (RB) and left border (LB). The borders are nicked by virulence protein VirD2 which produces single stranded transferred DNA (the “T-strand”) with covalent attachment of the VirD2 on its 5′ end. The protein-DNA complex, also including Agrobacterium VirE2 protein, exits Agrobacterium cells through the so-called Type 4 secretion system (T4SS, both virulence protein and ssDNA transporter), and is transferred into plant cells and integrated in the plant genome with the help of both Agrobacterium virulence proteins and plant factors. The use of Agrobacterium-mediated vectors to introduce DNA into plant cells is well known in the art. See, for example, the methods described by Fraley et al., (1985), Rogers et al., (1987) and U.S. Pat. No. 5,563,055, specifically incorporated herein by reference in its entirety.
Agrobacterium-mediated transformation is efficient in many dicotyledonous plants including Arabidopsis, tobacco, and tomato. Methods for Agrobacterium-mediated transformation of other species have also been devised (e.g. U.S. Pat. No. 6,384,301, relating to soybean transformation). While Agrobacterium-mediated transformation was at first only used with dicotyledonous plants, advances in Agrobacterium-mediated transformation techniques made the technique applicable to monocotyledonous plants as well. For example, Agrobacterium-mediated transformation techniques have been applied to rice (Hiei et al., 1997; Zhang et al., 1997; U.S. Pat. No. 5,591,616, specifically incorporated herein by reference in its entirety), wheat (McCormac et al., 1998), barley (Tingay et al., 1997; McCormac et al., 1998), and maize (Ishida et al., 1996). However, a number of plant species are recalcitrant to Agrobacterium-mediated transformation, and efficiency is low in others. Additionally, since A. tumefaciens enters plant tissues at wound sites, and does not naturally infect unwounded tissues, the use of certain tissues as transformation targets is not available.
Besides the T4SS-dependent T-strand delivery system, Agrobacterium has additional plasmid mobilization systems that can also transfer and integrate plasmids, such as the IncQ plasmid pRSF1010, between bacterial cells and into the plant genome with lower frequency via conjugal transfer (Buchanan-Wollaston et al. 1987, Shadenkov et al. 1996; Chen et al., 2002). For example, the conjugal transfer protein MobA, in conjunction with MobB and MobC proteins of the RSF1010 plasmid, cleaves the oriT (origin of transfer) site, attaches to the 5′ end and transfers the ssDNA into cells independent of the T4SS system (Bravo-Angel et al. 1999 and references therein).
Conjugal transfer systems are widely present in bacteria, resulting in exchange of genetic information between bacterial cells. In Rhizobium, phylogenetically related but distinct from Agrobacterium (Spaink, et al., (ed.), 1998; Farrand et al., 2003), the conjugal transfer system has been partially characterized in some species (Freiberg et al., 1997; Turner et al. 2002, Tun-Garrido et al. 2003, Perez-Mendoza et al. 2004). The conjugal system requires an oriT as the nicking site and TraA or Mob as a nicking enzyme, which is different from the conventional elements used in T-DNA mobilization (VirD2 and RB and LB sites, respectively). Unlike VirD2, which was found to have plant NLS (nuclear localization signal) at its C-terminus for plant nuclear targeting, the TraA or Mob doesn't have an obvious NLS. The precise mechanism and site of integration of DNA in plants by TraA remains unclear.
Members of the Rhizobiales other than Agrobacterium sp., such as Rhizobium spp., are known to symbiotically associate with plant roots in specialized nitrogen-fixing nodules (e.g. Long, 2001). In addition to host-specific nodulation of plant roots, especially of legumes, some plant growth promoting effects by members of the Rhizobiales are known in the absence of nodulation (e.g. Noel et al., 1996). Recently, reports have been published describing transformation of plants by bacteria other than Agrobacterium sp. (e.g. Broothaerts et al., 2005; U.S. Patent Application Publications 20050289667; 20050289672; Weller et al., 2004; Weller et al, 2005).
Broothaerts et al., reported transformation by Rhizobium sp., Mesorhizobium loti, and Sinorhizobium meliloti strains that was limited to Arabidopsis, tobacco, and rice. Weller et al. (2004, 2005) reported that several bacteria, including strains of Rhizobium sp. and Ochrobactrum sp. that harbored Ri plasmids apparently transformed hydroponically grown cucumber and tomato plants, leading to a hairy root phenotype. However the presence of Agrobacteria was not ruled out as a possibility in some inoculated plants, complicating the analysis. Transfer of DNA to soybean, corn, cotton, or canola plant cells by non-Agrobacterium bacterial strains has not been reported. In addition, transformation efforts in rice, tobacco, and Arabidopsis with non-Agrobacterium bacterial strains have to date been hampered by low transformation efficiencies. There is, therefore, a great need in the art for the development of improved methods allowing the transformation of important crop species using non-Agrobacterium bacterial strains, and improving transformation efficiencies in general.